Fluid powered actuators, whether they are hydraulic or pneumatic, are widely used for operating different pieces of equipment. In particular, these actuators may be used to operate valves on stationary machinery such as in complex pipe works in chemical plants.
As these actuators may control mixing valves, it is often necessary to know the exact position of the valve. This is typically achieved by monitoring the position of an actuator rod that is attached to the valve. A well-known solution is to install a position sensor, like a displacement linear transducer, on the outside of an actuator body, with a mobile end of the position sensor connected to the actuator rod so that the position of the valve is tracked.
However, because the actuators are commonly placed in areas that are exposed to harsh environments, the position sensor routinely fails. Moreover, because the actuators may also be installed in tight, difficult to reach areas, the replacement of the failed position sensor is cumbersome. Sometimes, the position sensor is so clogged with contaminants and the actuator so dirty, that it is necessary to completely remove the actuator since another position sensor cannot be installed in those conditions. The absence of the actuator generates equipment downtime, which translates into production, and therefore money, losses.
Different approaches have been suggested to solve this problem, mainly by protecting the position sensors from the environment. U.S. Pat. No. 7,290,476 to Glasson discloses a position sensor, such as an LVDT (linear variable displacement transducer), for a hydraulic cylinder. The position sensor is installed inside the cylinder, either on the cylinder bottom cap or on the cylinder head cap, and connected to the piston. Advantageously, the position sensor is protected from harsh environment. However, in case of failure of the position sensor, the whole actuator needs to be replaced since replacing the position sensor needs opening up the actuator.
Another solution has been proposed in U.S. Pat. No. 7,023,199 to Blubaugh et al. This patent discloses an actuator equipped with a position sensor that is fixed to the exterior of the cylinder bottom cap. The position sensor is of the magneto-restrictive type, which uses a pipe protruding inside the actuator, more precisely inside the center of the piston and rod. This type of sensor requires the use of magnets mounted on a mobile component of the actuator, such as the piston or the piston rod. An electronic module of the position sensor, mounted outside the actuator, is protected from the environment by a cover. A drawback of this design, though, is that the magnets may become contaminated, which then requires the disassembly of the actuator to access the magnets and replace them. This leads to downtime. Moreover, this type of sensor necessitates that the internal mobile components of the actuator, such as the piston and rod, be designed specifically to accommodate the pipe. This increases the price of the actuator. Furthermore, the operational temperature range of magneto-restrictive sensors is limited such that they may not work properly under certain environmental conditions.
There is therefore a need for an improved actuator capable of monitoring the position of its mobile components.